In order to determine the radioactive fixations image inside an organ, medicine makes use, among other means, of the principle of scintigraphy. This principle consists in introducing into the organism of a patient a radioactive element which will be more or less fixed on certain organs depending on whether these are healthy or unhealthy. The measurement of the intensity of emitted gamma radiation then provides an indication of the distribution of the radioactive element in the organism and therefore constitutes a radioactive aid. Such a measurement is carried out using a scintillation camera.
In conventional scintillation cameras, for example the Anger type camera (see U.S. Pat. No. 3,011,057), the gamma rays representing the radioactive distribution in the examined environment penetrate, after passing through a collimator, into a scintillator crystal. The scintillations which occur in the crystal are then detected by means of a whole series of photomultiplier tubes (for example 19, 37, etc. . . . ), after passing through a light guide providing optical coupling between the crystal and the tubes. These tubes are distributed in front of the optical unit (crystal+ light guide) in order to practically cover its entire surface and to transform the light energy of each appearing scintillation into a measurable electrical signal.
With each photomultiplier tube there is then associated an analog acquisition channel successively carrying out an amplification, an integration and a shaping of the signals provided-by the tube. The outputs S.sub.ij of the assembly of acquisition channels are sent to a computer which provides, by estimation, the coordinates x.sub.j and y.sub.j of a scintillation j and its energy E.sub.j, the index i indicating which of the acquisition channels is concerned. In the computer, several types of computing device can be provided, but essentially two of them are actually used, namely an arithmetic ratio barycentric computing device, or a logarithmic ratio barycentric computing device. In an arithmetic ratio barycentric computing device, the values x.sub.j, y.sub.j and E.sub.j are given by the equations: ##EQU1## In these expressions, we have: ##EQU2## with the coefficients G.sub.i, K.sub.i, H.sub.i, J.sub.i being weighting factors associated with the position of the axis of each of the p photomultiplier tubes.
In a logarithmic ratio barycentric computing device, the values x.sub.j, y.sub.j, E.sub.j are this time given by the equations: ##EQU3## in which the weighting factors are similarly associated with the position of the axis of each of the p photomultiplier tubes.
In the rest of the description it will be considered, without this being a limitation of the invention described later, that there is used for example an arithmetic ratio barycentric computing device.
The transfer of the output signals of the p acquisition channels to the computer determining the coordinates and the energy of the events j is very different according to whether these signals are analog, as in most of the conventional cameras, or digital, as in certain scintillation cameras of more recent design in which an analog-digital conversion of the signals is carried out in the acquisition channels.
When the output signals are analog, their transfer is carried out by means of resistor networks, whose ohmic value is proportional to the weighting factors, and summing amplifiers. An example of this type of signals transfer is described in the French Patent application, FR-A-2,288,987. According to this document, the objective of the described camera is to remedy the disadvantage which consists in using the output signals of all of the photodetectors for the localization of the scintillations. For this purpose, the camera includes means for selecting, from all of the output signals, a predetermined fixed number (three in the proposed example) of them, those of the photodetectors whose output signals are affected by a mediocre signal-to-noise ratio thus being eliminated. Because of this, the spatial resolution of the camera is improved.
Such a measure does not, however, enable in any way an improvement in the maximum counting of the camera. The applicant companies have in effect stated the limitations of the prior productions in terms of speed, particularly when the events to be detected are very close and when the scintillations, also very close in time, lead to at least a partial stacking of the electrical pulses corresponding to them.